As the detector for a gas chromatograph, various types of detectors have conventionally been proposed and practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD) and flame thermionic detector (FTD). Among those detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not sufficiently low. (2) Its ionization efficiency for alcohols, aromatic substances and chlorine substances is low. (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system more difficult to operate.
On the other hand, as a detector capable of high-sensitivity detection of various compounds from inorganic substances to low-boiling organic compounds, a pulsed discharge detector (PDD) has conventionally been known (for example, refer to U.S. Pat. No. 5,394,092). In the PDD, the molecules of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from the excited state to the ground state, they generate optical energy. This optical energy is utilized to ionize a molecule to be analyzed, and an ion current produced by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the FID for propane is no higher than 0.0005%, whereas the PDD can achieve a level as high as 0.07[%]. Despite this advantage, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is one or more digits lower than the latter. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, refer to U.S. Pat. No. 5,892,364). This detector uses a low-frequency AC-excited dielectric barrier discharge (which is hereinafter referred to as the low-frequency barrier discharge) to create a stable and steady state of plasma. The plasma created by the low-frequency barrier discharge is non-equilibrium atmospheric pressure plasma, which does not become hot so easily as the plasma created by the radio-frequency discharge. Furthermore, the periodic fluctuation of the plasma, which occurs due to the transition of the voltage application state if the plasma is created by the pulsed high-voltage excitation, is prevented, so that a stable and steady state of plasma can be easily obtained. Based on these findings, the present inventors have conducted various kinds of research on the discharge ionization current detector using a low-frequency barrier discharge and made many proposals on this technique (for example, refer to the following documents: International Publication No. WO2009/119050; Shinada et al., “Taikiatsu Maikuro-purazuma Wo Mochiita Gasu Kuromatogurafu You Ion-ka Denryuu Kenshutsuki (Excited Ionization Current Detector for Gas Chromatography by Atmospheric Pressure Microplasma)”, Extended Abstracts of 55th Meeting of Japan Society of Applied Physics and Related Societies in 2008 Spring; and Shinada et al., “Taikiatsu Maikuro-purazuma Wo Mochiita Gasu Kuromatogurafu You Ion-ka Denryuu Kenshutsuki (II) (Excited Ionization Current Detector for Gas Chromatography by Atmospheric Pressure Microplasma: Part II)”, Extended Abstracts of 69th Annual Meeting of Japan Society of Applied Physics in 2008 Autumn).
As explained previously, the low-frequency barrier discharge creates a stable plasma state and is also advantageous for noise reduction. However, it has the following problem.
In the discharge ionization current detector, the sample gas is normally mixed with a plasma gas and the ionization of the sample occurs in this mixed gas. In this process, it is preferable to supply the plasma gas at a high flow rate to improve the plasma's stability and the ionization efficiency. This is primarily because a higher flow rate of the plasma gas results in a larger amount of heat radiation from the electrode (which is heated by the plasma), thus preventing the electrode from being overheated and thereby damaged. The higher flow rate also contributes to a rapid removal of impurities released from the electrode and the inner wall of conduit lines, thus suppressing their influences. On the other hand, the plasma gas also acts as a diluent gas for the sample gas. From this point of view, the flow rate of the plasma gas should be lowered to improve the detection sensitivity of the sample components. Therefore, to make the detector applicable for various purposes, the flow rate of the plasma gas needs to be moderately selected to realize an appropriate trade-off between the plasma stability and the detection sensitivity. This means that the detector cannot be used for extreme cases, such as the detection of an extremely small amount of component.
In the case of a portable GC system designed for field analyses aimed at detecting volatile organic compounds (VOC) or similar substances, a small gas cylinder is used as the gas supply source. Therefore, the plasma gas needs to be supplied at the lowest possible flow rate during the detecting operation. On the other hand, when the measurement is performed for a high-concentration sample, lowering the flow rate of the plasma gas may lead to inadequate dilution of the sample, with the result that the sample's concentration falls outside the linear range of the detection sensitivity and hence cannot be correctly measured.
Thus, in the case of the conventional discharge ionization current detector, since the detectable range of the sample concentration significantly depends on the flow rate of the plasma gas, it is difficult for any single device to be used for various purposes and usages as well as for various samples having a broad range of concentrations, so that it is necessary to prepare a dedicated device for each different demand. The present invention has been developed in view of this point, and its objective is to provide a discharge ionization current detector applicable to a broad range of sample concentration and capable of performing an optimal measurement depending on the purpose, usage and conditions of the measurement.